The present disclosure relates to techniques of controlling an optical disk device for recording and reproducing information by irradiating a recording surface of an optical disk with laser light so that a tilt of an objective lens which is caused by a displacement of the objective lens is adjusted.
When recording and reproduction are performed on an optical disk using an optical disk device, the objective lens rolls due to tracking drive, so that the objective lens is tilted in a radial direction of the optical disk with respect to a recording surface of the optical disk. The tilt of the objective lens due to tracking drive is called an AC tilt. The AC tilt leads to a decrease in signal quality during recording and reproduction of information to and from the optical disk. Therefore, it is necessary to correct the AC tilt and thereby reduce or prevent the decrease in signal quality.
In order to correct the AC tilt, for example, the torsional rigidity of a coil spring which connects an optical pickup housing and the objective lens together may be increased. However, the increase in the rigidity leads to a decrease in drive sensitivity and therefore to an increase in heat generation and power consumption.
Alternatively, the center of gravity of the objective lens may be decreased by attaching a weight to a lower portion of the objective lens. However, the increase in weight of the objective lens leads to a decrease in responsiveness of the objective lens.
Therefore, in a conventional technique, focus position information and tracking position information are used to calculate the amount of tilt correction drive for correcting the tilt of the objective lens, whereby AC tilt correction is performed (see Japanese Patent Publication No. 2008-282439).
In another conventional technique, AC tilt correction is performed based on the number of tracks across which the optical pickup is moved without a tracking control (see International Publication WO2008/149557).
The technique of Japanese Patent Publication No. 2008-282439 has difficulty in ensuring the stability of AC tilt correction. Specifically, when the optical pickup passes a defective portion of an optical disk which is called black dot out (BDO), focus position information and tracking position information cannot be accurately extracted. Therefore, the amount of tilt correction drive is calculated using an equation of motion which governs the motion of an actuator, based on focus position information having an error with respect to correct focus position information and tracking position information having an error with respect to correct tracking position information. As a result, an error occurs in the amount of tilt correction drive. The necessity of calculation of the equation of motion increases the amount of calculation, compared to calculation of a sum of products. When the calculation is performed by hardware, the area of the hardware needs to be increased. When the calculation is performed by software, the software needs to be operated at higher speed. Therefore, the necessity of calculation of the equation of motion leads to an increase in cost of the optical disk device.
The technique of International Publication WO2008/149557 has difficulty in ensuring the following capability of AC tilt correction. After the amount of tilt correction is calculated based on the number of tracks across which the optical pickup is moved without a tracking control, the tracking control is started, and signal quality is evaluated during the start of the tracking control. Therefore, for example, if the signal quality does not reach a predetermined level, it is necessary to temporarily stop the tracking control in order to calculate the amount of tilt correction again. While the tracking control is stopped, at least one of reproduction and recording of information from and to an optical disk is not allowed, leading to an increase in the time required to activate the optical disk device.
The present disclosure describes implementations of a technique for AC tilt correction with stability and following capability in an optical disk device.